1. Technical Field
The present invention relates to variable speed power tools and more particularly, to variable speed power tools in which the operator may adjust the operating speed of the power tools. The variable speed power tools may be driven by various power sources, including but not limited to direct current, e.g. batteries, alternating current and compressed air.
2. Description of the Relevant Art
Some types of power tools are capable of opening in both a low speed mode and a high speed mode. For example, power polishers utilized to apply wax to a surface have a buff that preferably rotates at a low speed immediately after the wax is applied to the buff, so that the wax will not fly off into the surrounding areaand After the wax has become assimilated into the buff, the buff may rotate at a high speed. This high speed operation will improve the waxing efficiency of the buff.
In order to permit the operator to adjust the operating speed, variable speed power tools have been developed that are equipped with an operator adjustable speed adjusting device. In one type of variable speed power tools, the speed adjusting device has a position maintaining mechanism, which will maintain the adjusted position of the speed adjusting device, and therefore the operating speed of the power tool, until the operator again changes the setting position of the speed adjusting device. Representative examples of this type of speed adjusting device include (1) a dial that can be rotated by the operator and the dial will retain its position when the operator stops rotating the dial and (2) a sliding device that the operator can slide wherein the sliding device will retain its position after the position of the sliding device has been adjusted. Generally speaking, these known speed adjusting devices use variable resistors or potentiometers that adjust the operating speed by adjusting the current supplied to the power tool motor.
Power tools have also been developed that are equipped with a similar type of maximum speed adjusting device, which has a position maintaining function. In these types of known power tools, the operator can repeatedly adjust a speed adjusting device that is separate from the maximum speed adjusting device, which speed adjusting device is biased so that it will return to its original position after adjustment. Typically, the power tool will operate at higher speeds as the operator moves the speed adjusting device away from its original position. When the operator moves the speed adjusting device to the position that is furthest away from the original position, the maximum speed is obtained This maximum speed is determined by the adjusted position of the maximum speed adjusting device that incorporates the position maintaining mechanism, such as the above described dial or slide. Thus, when the speed adjusting device is in its maximum position, the maximum speed adjusting device solely determines the operating speed of the power tool.
When using a power tool equipped with a speed adjusting device having a position maintaining mechanism, the operator may set the desired operating speed. Thereafter, the operator is not required to constantly manipulate a switch or other device in order to maintain the desired operating speed. For other types of power tools equipped with a position maintaining mechanism and a separate speed adjusting device having a mechanism for biasing the speed adjusting device to return to its original position, a preset maximum speed can be obtained by merely adjusting the speed adjusting device to its maximum position.
Thus, the above-described power tools do not require the operator to further adjust the speed adjusting device or the maximum speed-adjusting device during operation and the power tool operation can be performed at the adjusted speed
However, if such known power tools are operated at the adjusted operating speed for an extended period and the operator must regularly change the operating speed to one or more operating speeds that are not regularly utilized, the operating speed adjusting operation can be burdensome. For example, when the operator uses a power polisher to polish a surface, operating the polisher at a high speed typically maximizes the polishing efficiency. However, it is necessary to apply more wax to the buff from time to time during this operation. After applying the wax to the buff, the operator must temporarily switch the polisher to a low speed mode, so that the newly applied wax does not fly off the buff Thus, the operator must adjust the speed adjusting device or the maximum speed-adjusting device from a high speed position to a low speed position.
However, the operator may have difficulty to quickly adjust the operating speed of the polisher before and after each application of the wax to the buff, because the speed adjusting device or the maximum speed adjusting device must be moved over a wide range to switch form the desired high speed to the desired low speed or from the desired low speed to the desired high speed. Further, it is especially difficult to quickly and reliably switch to the desired high operating speed after the wax has assimilated into the buff. Thus, if the operator determines a desirable operating speed that provides the highest waxing efficiency, it may be very difficult to locate that same desired high operating speed after applying more wax and operating the buff at a low speed to assimilate the wax into the buff.
Thus, it is one object of the present invention to teach improved power tools that overcome some or all the above-mentioned problems with known variable speed power tools.
In one aspect of the present teachings, power tools are taught that can temporarily switch from a first operating speed, such as a maximum operating speed, to a second operating speed, such as a minimum operating speed
In another aspect of the present teachings, variable speed power tools are taught that have a fixed operating speed switch and an operator adjustable speed adjusting device having a position maintaining mechanism. When the fixed operating speed switch is closed, the operating speed of the power tool changes to an operating speed that may be, for example, preset by the operator or the manufacturer. If such a fixed operating speed switch is included in the power tool, the operator can easily change to a preset operating speed that is appropriate for certain operations. In that case, the operator is not required to adjust the position of the speed adjusting device to a position that corresponds to the preset operating speed Rather, simply by switching the fixed operating speed switch, it is possible to temporarily change to the appropriate preset operating speed Further, by disengaging or turning off the fixed operating speed switch, it is possible to return the power tool to the operating speed that corresponds to the position of the speed adjusting device. Because the speed adjusting device remains in the same position until moved by the operator, the power tool will be returned to the operating speed previously selected by the operator when the fixed operating speed switch is disengaged or turned off.
In another aspect of the present teachings, the power tool may include a preset maximum speed switch in addition to an operator adjustable maximum speed adjusting device having a position maintaining mechanism and an operator adjustable speed adjusting device having no position maintaining mechanism. This type of power tool operates at its maximum speed when the operator adjusts the speed adjusting device to its full or maximum position, and the maximum speed so adjusted corresponds to the position of the operator adjustable maximum speed adjusting device. When the preset maximum speed switch is turned on, the maximum speed realized when the operator adjusts the speed adjusting device to its full or maximum position changes from the maximum speed corresponding to the position of the operator adjustable maximum speed adjusting device to a preset maximum speed that may be, for example, preset by the operator or the manufacturer. In this case, the operator is not required to adjust the position of the operator adjustable maximum speed adjusting device in order to temporarily change to the preset maximum speed.
In another aspect of the present teachings, power tools may include a fixed operating speed switch in addition to an operator adjustable maximum speed adjusting device an operator adjustable speed adjusting device. This type of power tool operates at its maximum speed when the operator adjusts the speed adjusting device to its full or maximum position, and the maximum speed so adjusted corresponds to the position of the operator adjustable maximum speed adjusting device. When the fixed operating speed switch is turned on, the operating speed of the power tool changes to the preset fixed speed that is previously chosen by the operator or the manufacturer. When the fixed operating speed switch is turned off, the operating speed of the power tool changes to the speed that is set by the adjustable positions of the operator adjustable maximum speed adjusting device and the operator adjustable speed adjusting device.
In another aspect of the present teachings, power tools may include a control circuit that adjusts the operating speed to match a target speed using feedback control. The target speed may be either the operating speed set by the speed adjusting device or a preset operating speed determined by the fixed operating speed switch. In one representative embodiment of this aspect, a voltage corresponding to the target speed is coupled to one input pin of the control IC and a voltage corresponding to the actual operating speed is input to another input pin of the control IC. A differential amplifier, or other similar circuit, may compare the two voltages to determine whether the actual operating speed should be increased or decreased in order to match the target speed For example, an ON signal inputted to a semiconductor device that controls the operating speed of the motor is delayed when the actual operating speed is higher than the target speed.
As a result of the present teachings, the operation of temporary switching to a previously determined operating speed or the operation of switching from the previously determined operating speed to the operating speed corresponding to the position of the speed adjusting device can be simplified and operating efficiency can be improved. Further, power tools of the present teachings can be realize at a low cost.
Throughout this specification, the term xe2x80x9ca speed adjusting device having a position maintaining mechanismxe2x80x9d is intended to mean any kind of device that can be utilized to control the operating speed of the power tool, which device will retain its setting until the operator again adjusts the device. Thus, in power tools equipped with such a speed adjusting device having a position maintaining mechanism, once the operator has set the speed adjusting device to the position that corresponds to the appropriate or desired operating speed, the operating speed will not change until the operator again changes the setting. Thus, the operator can perform the power tool operation at the set operating speed without further activity.